The long-term objective of this project is a better understanding of the role of genetics in human behavioral disorders. The particular disorder to be studied initially is stuttering because it is intrinsically interesting, it is a common disorder in children, it is relatively easy to diagnose, and recent analyses have suggested that a single major gene locus may be involved. A new analytic method based on sex effect has suggested the design for a family study. I propose to collect data on the relatives of stutters. Both children and adults will be used as probands and information on all speech disfluencies in their first degree relatives will be sought by personal interview. After classifying probands and their relatives by sex and/or severity, the average risk for each type of sex-specific relationship will be calculated. These risks will be analyzed by the new methods based on sex effect as a two-threshold model; unambigous discrimination between the two genetic models - multifactorial and single-major-locus - should be possible.